1. Field of the Invention
The present invention relates to a composite video signal combining system by which separately produced composite video signals representing a trichromatic foreground scene, such as a person picture in front of a backing, and a trichromatic background scene may be combined electronically to produce an output composite video signal representative of a resulting combined picture.
2. Description of the Prior Art
This invention pertains more particularly to a composite color picture combining method for which the foreground picture, as a person, is initially filmed before a trichromatic backing. Relatively to a primary colorimetric coordinate system, the trichromatic region of the backing is enveloped by the trichromatic region representative of the foreground. A second video source, such as a camera, films the background scene such as a landscape.
This method is disclosed in the U.S. Pat. No. 3,764,732. The composite video signal combining system according to this U.S. Patent produces primary control signals from separated component video signals forming the composite video signal representative of the foreground scene. For each component video signal the primary control signal is a logic signal having two levels. A first level or low level corresponds to picture points of the foreground picture (or person). The first level is produced when the amplitude of the corresponding foreground component video signal does not lie within a reference voltage range, the voltages of which define at least two backing picture points. The second or high level is produced when the amplitude of the corresponding foreground component video signal lies within the corresponding voltage range. For example, for the red R, green G and blue B primary color system, the backing is defined by a trichromatic region having three reference voltage ranges relative to red, green and blue colors.
Then the three primary control signals are multiplied into a control signal. The first level of this signal is produced for picture points belonging to the foreground picture (or person) and the second level is produced for picture points belonging to the trichromatic backing. Input means are provided for receiving the foreground and background composite video signals. Switching means are controlled by the control signal to cause the crossing of the foreground composite video signal representative of the person picture through the foreground composite video signal receiving input means when the control signal is at the first or low level and to cause the crossing of the background composite video signal through the background composite video signal receiving input means when the control signal is at the second or high level. The backing is also replaced by the background scene when the control signal is at second level. The resulting picture is obtained by mixing or combining of the composite video signals outputting from input means. The resulting picture, therefore, contains the foreground person in front of the background scene, such as a landscape.
According to the U.S. Pat. No. 3,764,732, the first and second levels of the primary control signals or the control signal are separated by sharp rise and fall fronts, such as logic signals. Consequently, the edge between the moving foreground picture and the background scene picture which replaces the backing, in the resulting composite picture is very clearly defined which does not make it possible to achieve transparency effects. Indeed, when the moving foreground picture includes a transparent or partially transparent object, such as a glass or a bottle, filmed by a first camera, the resulting composite picture presents the backing through the object's transparency, and to a large extent no longer lies within the trichromatic region of the backing. During picture combining, instead of seeing the background scene image, such as a landscape, through the transparent object, one sees the backing through the object. The same phenomenon can occur when the moving foreground picture is a person whose hair is dishevelled. Other composite video signal combining systems are known in which the moving foreground picture is initially reproduced or recorded before a backing which has a selected primary color of high saturation, generally the blue color; these known process are ordinarily referred to by the customary term "blue screen or backing process". With this in mind, reference could be made to U.S. Pat. Nos. 3,778,542; 3,595,987 and 4,007,487, and German Patent Application No. 2,749,254 (claiming U.S. Patent Application priorities Ser. No. 73 8740 of Nov. 3, 1976 and Ser. No. 83 8097 of Sept. 30, 1977).
According to these systems with unicolor backings, the backing is restricted by an open segment along one of the coordinate axes of the trichromatic coordinate system R, G, B, from a determined lower threshold. The picture combination controlling signal is obtained by the selection of the determined threshold, which varies as a function of the amplitude of the selected foreground component video signal, such as the blue component, and which is balanced by a linear relation of one or both of the other two foreground component video signal (red and/or green). The drawback of this balancing is that it must be set manually for each series of foreground scene pictures depending on the prevailing colorimetric tonality of the latter. However, it does make it possible, in certain cases, to visualize the background scene picture through a transparent object of the foreground picture, but with a certain blur which can be adjusted manually. Furthermore, relative to the selected backing color, the moving foreground picture (person or analogous) must not have any picture points with a corresponding primary color component whose amplitude is greater than the sole one of the backing. In other words, the foreground scene picture's (such as a person) trichromatic region is limited by a movable plane which is parallel to the coordinate plane of the other primary color components (red and green) and which cuts the selected primary component's (blue) coordinate axis at a point whose coordinate is equal to the lower threshold of the backing's primary component. The region containing the origin of the trichromatic system and limited by the above-defined plane is that of the foreground (picture part to be included in the resulting picture).